ATR-probes include, usually, a radiation guiding body, in which light is guided by means of total reflection at a surface of the body, wherein the surface is contactable with the medium to be examined, whereby the evanescent field of the light interacts with the medium.
The radiation guiding body is usually arranged at a media opening of a probe housing, which it closes, and through which a surface of the radiation guiding body is contactable with the medium.
A probe includes, furthermore, a light source for coupling light into the radiation guiding body and a detector for transducing the light coupled out of the radiation guiding body into an electrical signal, as well as, should the application require, light conductors for conducting light from the light source to the radiation guiding body or from the latter to the detector.
A challenge for the design of these probes is presented by the in- and out-coupling of the light into and out of the radiation guiding body, for, in the ideal case, each ray should, on the one hand, undergo a number of total reflections at the bounding surface in contact with the medium, and, on the other hand, the construction should be simple. The simplest construction would be a planparallel plate, between whose planparallel surfaces the light can be multiply guided by means of further total reflections. Here, however, there is the problem of the in-coupling. Diamond is the most suitable material for the radiation guiding body. Due to the high material price, however, complex optical designs, which consume a large volume of material, or require many facets, are too expensive.
Sting and Milosevic disclose, therefore, in U.S. Pat. No. 5,703,366 an ATR-probe, wherein a coupling body, for example, of ZnSe, sapphire or the like, and having an annular coupling surface, is placed on the radiation guiding body, so that a direct transferring of the light between ZnSe and the diamond of the radiation guiding body is accomplished. The coupling body has inclined deflection surfaces, which enable it to couple the light at such angles into the radiation guiding body that, on the bounding surface facing away from the radiation guiding body and facing the medium, total reflection is brought about. In order to enable multiple reflections between the planparallel surfaces of the radiation guiding body, the coupling body has in its center a cavity surrounded by the annular coupling surface, so that light rays totally reflected from the media-contacting bounding surface cannot be coupled out in the region of the cavity, but are, instead, totally reflected anew to the media-contacting bounding surface. This construction is, however, very complicated, since the interface between the coupling body and the radiation guiding body is susceptible to disturbances, for example, on the basis of pressure- and temperature fluctuations, which can strongly degrade the effectiveness of the coupling.
German Patent, DE 10 2006 036 409 F1 discloses an ATR-probe, which eliminates the above problem, using a radiation guiding body having planparallel surfaces, which has, on the outer edges of the surfaces, pairwise oppositely lying, facet-like deflection surfaces, wherein light coupled in essentially perpendicularly to the surfaces at a first facet is deflected in such a manner that it, after a total reflection about in the center of the media-contacting surface, hits the second facet and is there deflected anew, in order then to be coupled out essentially perpendicularly to the planparallel surfaces of the radiation guiding body.
Although this approach quite easily achieves the desired simplicity of construction and the desired insensitivity, it is, nevertheless, achieved with a smaller sensitivity of the ATR-probe, for the ray undergoes only one reflection at a comparatively large angle on the media-contacting, bounding surface, so that the interaction of the evanescent field with the medium, which lastly determines the sensitivity of the probe, is comparatively slight.